The Kraft process has been in existence since 1879 and is the dominant wood pulping process today. This is due largely to the comparative simplicity and rapidity of the process, its insensitivity to variations in the wood condition, and its applicability to all wood species, as well as the valuable properties of the wood pulp produced.
In the Kraft process, described in more detail below, wood chips are cooked in white liquor and washed to make pulp. The white liquor is comprised of NaOH and Na.sub.2 S. The residue of the washing, black liquor, contains alkali lignin, hydrolysis salts, and sulphonation products. Through a number of other processes, including a chemical recovery furnace, the black liquor is regenerated into the white liquor that cooks the wood chips. The economics of the Kraft process depends on the efficiency of the recovery of the black liquor and the regeneration of the white liquor.
Now referring to Prior Art FIG. 1, there is shown a schematic of the Kraft process 10 to further explain its details. In the Kraft process 10, wood chips 40 are placed into a digester and blow tank 12 and cooked with white liquor stream 42 to start the pulping process. White liquor stream 42 is made up of NaOH and Na.sub.2 S in aqueous solution. The cooking of the wood chips 40 with the white liquor stream 42 results in a black liquor and pulp stream 44 that is transferred to a washer 14 and washed with wash water 46. This is the start of the black liquor processing section 13 of the Kraft process.
The washer 14 separates the black liquor and pulp stream 44 into a pulp stream 48, which is used to make paper, and a weak black liquor stream 50 comprising about 12 to 15% by weight solids. The weak black liquor stream 50 stream is transferred to oxidizer 16 and oxidized. The oxidized weak black liquor stream 51 is then transferred to the multiple effect evaporator 18 and concentrated to approximately 45 to 50 weight percent solids to become a black liquor stream 52. The black liquor stream 52 is then transferred to forced circulation concentrators 20 where it is concentrated to about 65 weight percent solids. The resulting concentrated black liquor stream 54 is then mixed with make-up Na.sub.2 SO.sub.4 56 and directed to a recovery furnace 22, the last element of the black liquor processing section 13.
The recovery furnace 22 of the Kraft process 10 produces a smelt stream 60 that is mixed with a weak white liquor stream 80 in a dissolving tank 24 to produce a green liquor stream 62. The green liquor stream 62, which is comprised of Na.sub.2 S and Na.sub.2 CO.sub.3, is transferred to a green liquor clarifier 26, where dregs 64 are removed. A clarified green liquor stream 63 is then directed to a slaker and causticizer 28, where it is mixed with a quick lime stream 78. The resulting mixture, an unclarified white liquor stream 41, is transferred and clarified in a white liquor clarifier 30. The sediment in the unclarified white liquor stream 41 is removed from the white liquor clarifier 30 as a lime mud stream 68. The clarification also produces the white liquor stream 42, that is directed to the digester and blow tank 12, thus completing the circuitous liquor route of the Kraft process. The white liquor clarifier 30 and the white liquor stream 42 comprise the white liquor processing section 15 of the Kraft process.
The lime mud stream 68 is further processed to produce the weak white liquor stream 80 and a quick lime stream 78. The lime mud stream 68 is directed from the white liquor clarifier 30 to a mud washer and filter 32. In the mud washer and filter 32, the mud is washed with a water stream 70 to produce a washed lime mud stream 66 and the weak white liquor stream 80. As described previously, the weak white liquor stream 80 is directed to the dissolving tank 24.
The washed lime mud stream 66 is further processed in a lime kiln 34. In the kiln 34, the washed lime mud stream 66 is replenished with a make-up CaCO.sub.3 stream 72 and the mixture is calcined to produce the quick lime stream 78 and a kiln emissions stream 76. The quick lime stream 78 is directed to the slaker and causticizer 28. The calcining process contributes CO to the kiln emissions stream 76.
The lime kiln 34 also receives a non-condensible gas ("NCG") stream 58 from an NCG collection system 36. The NCG collection system 36 receives NCGs from plant wide sources 74. Plant wide sources 74 comprise all of the locations that generate NCGs, including the head spaces in tanks and other pieces of equipment, the fumes released by the digester and blow tank 12, and any other fumes collected by the NCGs collection system 36. The NCGs are comprised of SO.sub.2, C.sub.10 H.sub.16 (.alpha.-pinene and .beta.-pinene), H.sub.2 S, CH.sub.3 SH, (CH.sub.3).sub.2 S and (CH.sub.3).sub.2 S.sub.2. In the lime kiln 34, the SO.sub.2 in the NCGs reacts with Ca and O.sub.2 to form CaSO.sub.4, or the SO.sub.2 reacts with Na.sub.2 CO.sub.3 to form Na.sub.2 SO.sub.4.
The lime kiln 34 burns at least a portion of the partially volatile organic compounds (VOCs) in the NCG stream 58 and another portion may react with the lime in the kiln. Any of the unreacted or uncombusted NCGs are released with the kiln emissions stream 76. If the lime kiln is down, the NCG stream 58 will necessarily be released unprocessed. A scrubbing system (not shown) may be used to scrub the CO.sub.2 from the kiln emissions stream 76. Kraft processes typically use rotary lime kilns, though some use a fluidized bed. See G. A. Smook, Handbook for Pulp & Paper Technologists, p. 142-145 (1987), which is incorporated by reference herein in its entirety.
Increasingly stringent environmental regulations restrict chemical emissions from pulp mills. The regulated chemical emissions include sulfur dioxide (SO.sub.2), .alpha.-pinene (C.sub.10 H.sub.16), .beta.-pinene (C.sub.10 H.sub.16), hydrogen sulfide (H.sub.2 S), methyl mercaptan (CH.sub.3 SH), dimethyl sulfide ((CH.sub.3).sub.2 S) and dimethyl disulfide ((CH.sub.3).sub.2 S.sub.2). Chemical analyses of the pulp mill emissions show that the regulated chemicals are present in concentrations ranging from about 500 ppmv to 15,500 ppmv in the NCG stream 58.
To achieve satisfactory emissions, based on 40 C.F.R Chapter 1, Subpart BB--Standards for Performance for Kraft Pulp Mills, a Kraft process's emissions must meet the following criteria: EQU SO.sub.2 .ltoreq.15 ppmv EQU H.sub.2 S.ltoreq.5 ppmv EQU CO.ltoreq.30 ppmv
Attainment of these criteria generally require total hydrocarbon and total reduced sulfur (TRS) compound destruction and removal efficiencies of .gtoreq.99.99% and SO.sub.2 removal efficiencies of .gtoreq.98%. Approaching and exceeding these destruction and removal efficiencies enhances odor control since the concentration of odorous compounds is reduced closer to their odor thresholds.
A number of methods have been used to achieve satisfactory emissions, as explained in T. L. C. De Souza, Controlling SO.sub.2 Emissions from Combustion Sources, 1994 International Environmental Conference, TAPPI Proceedings, p. 583-589. One method is to optimize the operating conditions in the chemical recovery furnace 22. It has been found that SO.sub.2 emissions can be reduced by providing increased O.sub.2, efficient air mixing turbulence, and correct bed temperature in the chemical recovery furnace. Mr. de Souza reports that there are several low odor Kraft recovery furnaces that are optimized to achieve SO.sub.2 emissions as low as 20-25 ppm.
Another method for reducing emissions is to oxidize the weak black liquor stream 50 prior to it being reduced into strong black liquor stream 54 by direct contact evaporators. See, Don. H. Padfield, Control of Odor from Recovery Units by Direct-Contact Evaporative Scrubbers with Oxidized Black Liquor, TAPPI, Vol. 56, No. 1, p. 83, 86 (January 1973)(reporting that an evaporative scrubber was used to reduce SO.sub.2 emissions 50-80%).
Another method of reducing emissions is by scrubbing the NCG stream 58 with an alkaline solution. The removal efficiency for a venturi/cross-flow scrubber system was reported at 90% for H.sub.2 S, SO.sub.2 and CH.sub.3 SH, with 70-80% removal for (CH.sub.3).sub.2 S and (CH.sub.3).sub.2 S.sub.2. The scrubbing results in S.sub.2 O.sub.3 and SO.sub.4 salts in the liquid stream leaving the scrubber, which is returned to the Kraft process to be reduced to Na.sub.2 S. De Souza at 584.
Another method to reduce emissions is to wash Na.sub.2 S from the lime mud stream 68 prior to calcination in the kiln 34. The Na.sub.2 S in the lime mud stream 68 can become H.sub.2 S when exposed to CO, and the H.sub.2 S can be oxidized to SO.sub.2. By reducing the Na.sub.2 S in the lime mud stream, the amount of SO.sub.2 in the kiln emissions stream 76 of the kiln is reduced.
Another method to reduce emissions is to dry scrub the emissions from power boilers (not shown) or to scrub the power boiler emissions with an oxidized weak black liquor or NaOH. See, C. I. Harding and S. F. Galeano, Using Weak Black Liquor for Sulfur Dioxide Removal and Recovery, TAPPI, Vol.51, No. 10, p. 48A (October 1968).
The lime kiln 34 also reduces the emission of sulfur from the Kraft process. In treating the NCG stream 58, the lime mud/quick lime in the lime kiln absorbs and reacts with a portion of the SO.sub.2 present in the NCGs to form CaSO.sub.4 and Na.sub.2 SO.sub.4. However, the absorption reaction does not reduce the SO.sub.2 in the gaseous product stream 76 of the lime kiln 34 sufficiently. To further reduce SO.sub.2 in the stream 76, a scrubber is installed (not shown). The scrubber may use an NaOH solution. Due to the sulfur load on a lime kiln 34, the stream 76 may need to be treated by both an electrostatic precipitator and a scrubber. Erkki Kiiskila, Lime Kiln Emission Control, TAPPI Proceedings of 1990 Annual Meeting, Atlanta Ga., p. 121.
Much attention has also been given to methods of reducing odor emissions from the Kraft process. Marshall Allen, Robert Wilbourn, and David Wright, Flameless Thermal Oxidation for Odor Control, presented at the "Incineration Technology" conference, IBC Technical Services Ltd., Environmental Division, Manchester, England (Oct. 24-26, 1995). An easy method is to use pleasant odorants to mask the offensive odor. Another method is to alter the chemical process, as described previously, and reduce the emissions of the offensive chemicals. Atlernatively, the offensive emissions can be adsorbed with activated carbon and zeolites, or scrubbed. Scrubbing is effective for removing the SO.sub.2 in the emissions, but theVOCs that create an offensive odor, such as C.sub.10 H.sub.16, CH.sub.3 SH, (CH.sub.3).sub.2 S, and (CH.sub.3).sub.2 S.sub.2, if removed, are merely in another form that requires further treatment.
Tighter regulatory controls of VOCs emissions have resulted in an emphasis on destructive methods to convert VOCs to benign or easily treatable compounds. Baseline technologies in this group include wet chemical oxidation and bio-treatment. While the baseline technologies have found limited application in odor reduction, the reaction kinetics are generally slow, thus restricting their broad use in process odor control. Conventional treatment technologies, e.g., flares and incinerators, also fall within this group of baseline technologies. Flares and incinerators are widely used to treat VOCs emissions. However, their continued use is uncertain due to a lack of public acceptance and general inability to meet tightening emission control regulations.
New technologies have been developed to treat odor control. Catalytic oxidation has been used effectively to control odor. However, many of the chemical constituents that are problematic from an odor standpoint, e.g. sulfur, tend to poison catalysts. Regenerative thermal oxidation is capable of processing streams containing VOCs but destruction and removal efficiencies below 95% are common. Since odor thresholds are generally a function of concentration, the greater the destruction and removal efficiency, the greater the odor reduction. To achieve this sufficient odor control, it has been determined that the destruction and removal efficiencies need to be greater than 99.99%.
Variable emissions add to the complexity of treating the NCG stream 58. The Kraft process and the additional systems associated with the Kraft process can generate low volume, high concentration emissions and high volume, low concentration emissions. The overall composition of the emissions at any one time varies widely, demanding a robust system to perform the necessary reductions.
Thus, there is a need for a system and method for reducing the emissions and odors of the emissions of a Kraft process to meet environmental regulations. Further, there is a need to recover the sulfur from Kraft process emissions back into the Kraft process. Additionally, there is a need to handle the variable emissions of a Kraft process.